Measuring cable travel sensors are typically used in a very wide range of different situations in order to be able to precisely ascertain he instantaneous position of a remote body or component at any time. The remote body or component which is variable in respect of its position may be for example the rudder of a tailplane assembly on an aircraft or for example the cabin of an elevator.
For that purpose a typical form of measuring cable travel sensor comprises a measuring cable which is wound onto a cable drum and to the free end of which the movable body whose position is to be detected and monitored is secured, for example by being secured to a cable eye at the free end of the cable. The measuring cable travel sensor is disposed at a stationary position for monitoring the movement of the movable body with respect there to.
The above-mentioned cable drum is so pre-stressed for example by means of a spring that the measuring cable is always wound onto the cable drum as far as possible. The position of the remote movable body is ascertained by any rotary movement or part of a rotary movement of the cable drum being detected by means of a rotary position sensor whereby the instantaneous position of the movable body whose position is to be monitored can be known at any time by cumulation of the signals from the rotary position sensor.
So that the position of the movable body can be determined with a very high degree of accuracy, the measuring cable which is wound on the cable drum is a cable or wire that is as thin as possible, and it is wound on the cable drum in a defined manner, in particular in the form of only a single winding or layer of turns on the periphery of the cable drum as, if the cable were to start to wind over the top of the winding on the periphery of the drum, in such a way as to form a second layer of turns thereover, that would increase the circumference of the cable winding on the drum and that would therefore falsify the measurement result. For that purpose the diameters of the cable drums are selected to be as large as possible in relation to the diameter of the measuring cable.
For the same reason evaluation of the rotational movements of the cable drum, which is generally effected by opto-electronic or electrical means, must be kept as free as possible of any influences which could cause falsification of the measurement result, that is to say on the one hand any kind of fouling or contamination, and in particular moisture, and on the other hand electromagnetic radiation, which therefore already requires a considerable level of expenditure to obviate such adverse effects.
In addition there are also situations of use of measuring cable travel sensors, in which the movable body whose position is to be monitored by the sensor is not operating under normal pressure, but under a differential pressure, generally a relatively high overpressure. In that situation it is often not possible for the cable eye of the measuring cable, by means of which the measuring cable is secured to the moving body, to be arranged on the moving body in such a way that the fixing point established by the cable eye can be adequately sealed relative to the differential pressure environment in which the moving body whose position is to be determined by the sensor is disposed.
That accordingly involves a requirement to provide a measuring cable travel sensor which is also capable of operating under conditions involving a differential pressure and in particular an overpressure. In that respect in particular it must be possible to arrange the cable eye of the measuring cable in a space which is under such a differential pressure, that is to say generally an overpressure.
In such a situation, without additional measures being taken, the fact that the cable guide bush through which the measuring cable passes into the housing of the measuring cable travel sensor is not pressure-tight would mean that the overpressure which obtains in the region of the cable eye would therefore also prevail in the interior of the housing of the measuring cable travel sensor and there initially in the region of the cable drum which is to wind on the measuring cable.
In measuring cable travel sensors of that kind however the spring for prestressing the cable drum is generally arranged in axially displaced relationship with respect thereto, and the cable drum and the spring are non-rotatably connected to each other by way of a shaft. The rotary position sensor for sensing the rotary position of the cable drum is additionally arranged on that shaft, at the side of the spring.
That design configuration provides an axial succession from the cable drum by way of the spring to the rotary position sensor and the electronic evaluation system to which the rotary position sensor is connected, and that is specifically adopted so that the fouling or contamination which inevitably occurs at the cable drum, due to the measuring cable being movable between positions in which it is first outside and then inside the housing of the measuring cable travel sensor, can also reach the spring and the rotary position sensor to an ever decreasing degree.
For that purpose the typical design configuration of such a measuring cable travel sensor also includes axial sealing means for sealing the cable chamber in which the cable drum is disposed, with respect to the axially adjoining spring chamber which accommodates the spring and the sensor chamber which accommodates the rotary position sensor. In that respect a particularly important consideration is to provide for axial sealing integrity at the location at which relative movement occurs, that is to say where the central shaft connecting the cable drum and the spring is passed through structure such as a housing wall. Known shaft sealing rings, radial lip seals, O-rings or the like are generally used for that purpose, depending on the respective situation of use involved.
If however shaft seals of that kind are under an overpressure at one side, which would be the case in the event of a differential pressure obtaining in the cable chamber accommodating the cable drum, then such seals apply to the shaft a generally greatly increased radial force which is very greatly different from the normal condition.
In a typical prior measuring cable travel sensor however that would mean that the measuring cable would not wind onto the cable drum, under the effect of the prestressing force, in the correct manner, for example not in the correct timed relationship, but in a slightly time-displaced fashion, and that is unacceptable for the situations of use involved which are generally highly critical in terms of position. In particular that situation could result in the cable jumping over the winding on the cable drum, that is to say it could result in a second winding being formed over the top of a first layer of turns on the drum, with the structure of the second layer of turns being undefined and therefore catastrophic in terms of the accuracy of the measurement result.
Furthermore the cable chamber accommodating the cable drum on the one hand and the sensor chamber accommodating the rotary position sensor on the other hand cannot simply be completely open relative to each other as otherwise the fouling and contamination which generally occurs at the cable drum would also occur equally at the rotary position sensor and would thus have a long-term adverse effect on the measurement result thereof.
It will be appreciated at this point that the most obvious option in this respect is for the location at which the cable passes into the housing of the measuring cable travel sensor, for example by passing through a wall thereof, to be of such a design as to afford sealing integrity, for example by means of a plurality of successively connected labyrinth seals, or the like. However, because of the sliding friction which would occur at such an arrangement and the wear that this would entail that is not a solution which in the long term gives satisfactory results, in particular also for the reason that the measuring cable used is generally a braided steel cable whose outside peripheral surface is accordingly not smooth but is of a somewhat corrugated configuration by virtue of the braiding structure.
Furthermore an important requirement on the part of the user is that it must be possible to carry out maintenance and repair operations in particular on the rotary position sensor and the electronic evaluation system connected thereto without having to remove the differential pressure in the region of the cable eye at which the movable body is connected to the measuring cable.